Positioning stages, as that term is used herein, are devices for moving a table with respect to a frame within at least one and usually more than one predetermined degree of freedom by a drive through a motion transmission system so that the table can be placed in a fixed predetermined position with respect to the frame. An example of a positioning stage is described in U.S. Pat. No. 4,766,465.
Highly refined positioning stages are well known for optical microscopes, specifically for moving a work piece mounting table along X and Y axes to bring the work piece to the desired predetermined position in the optical system. Examples of microscope positioning stages are disclosed in U.S. Pat. Nos. 5,077,620, 5,000,554 and 4,824,229.
It is well known to sense changes of relative position between two movable parts by the use of electronic encoders which send a signal, typically a quadrature encoded signal, to a microprocessor to indicate when changes of position between the two parts have occurred. The microprocessor then activates one or more drive mechanisms to return the parts to the predetermined fixed position or to move them to a different predetermined fixed position. Encoder pairs are described in each of the three patents identified above. In each of those instances, and in all other known designs, encoder pairs sense relative movement within the parts of one or more of the drive motors or of the motion transmission systems such as gear trains.
In every such case there is a probability of some lost motion in the motion transmission system between that sensing point of the encoder pairs and the table which requires positioning, and it could result in undetected and therefore uncontrolled motion of the table. To guard against this undesirable result prior art designs have uniformly resorted to high precision gearing and the use of relatively expensive stepped electric motors, all of which are carefully crafted to eliminate play, slop or other lost motion in the motion transmission system between the place of operation of the encoder pair and the table to be positioned. Inexpensive small direct current electric motors, non-precision hydraulic motors or cylinders and loose tolerance gear trains have not been employable on microscopic positioning stages or any other positioning stages where relatively fine positioning is required for the work table.
It is the principal purpose of the present invention to improve upon positioning stages so that encoder pairs may accurately fix a work piece on a table in a predetermined position irrespective of lost motion in the transmission system between the drive of the stage and the work table. This object is to be achieved even when the work table operates within a plurality of degrees of freedom with a plurality of motion transmission systems involved. Specifically, it is the purpose of the invention to provide a microscope positioning stage using inexpensive electric or hydraulic devices and gear trains which incorporate encoder pairs and microprocessors in a manner such that a work table can be positioned within an accuracy of five microns plus or minus.